1. Technological Field
This technical disclosure pertains generally to functional electrical stimulation and electrophysiological recording devices for biomedical applications, and more particularly to recording immediate electrophysiological responses during or after stimulation.
2. Background Discussion
Recording during or immediately after electrical stimulation of neural tissues allows researchers to investigate the sophisticated dynamics of neural networks for future prosthetic devices and biomedical implants. However, the recorded neural responses are often obscured by large stimulation artifacts. Stimulation artifacts arise due to either current or voltage stimulus passing through the electrode-tissue interface. The largest artifact usually appears at the site of the stimulation electrode with degraded intensity in surrounding tissues. In one example, a 100 μA current stimulus injecting into an electrode of 10k ohm impedance resulted in an artifact of approximately 1 V at the electrode and attenuated at sites away from the electrode position. Thus, removing and minimizing the artifact is important to reliably and faithfully record and analyze the neural signal.
Approaches have been attempted for using software-based methods to recover the neural signal; however, these methods tend to be time consuming. More problematic is that the artifact amplitude is typically larger (often significantly) than the neural signal of interest, so that if the neural signal amplifier provides even a moderate gain, a severe saturation of the amplifier can result because of the presence of artifacts. Once the amplifier is saturated, the neural signal is lost and cannot be recovered with software-based post-processing.
In addition, even if the amplifier is not saturated in the process, the neural signal of interest is often attenuated/distorted due to the gain desensitization of an amplifier. The gain of an amplifier is desensitized when a larger interference is presented together with the small signal of interest. For illustration, given an amplifier as a linear-time-invariant system with input of x(t) and output of v(t), v(t) this can be expressed as follows:
                                                                        Y                ⁡                                  (                  t                  )                                            =                            ⁢                                                                    ∂                    1                                    ⁢                                      (                                                                                            X                          1                                                ⁡                                                  (                          t                          )                                                                    +                                                                        X                          2                                                ⁡                                                  (                          t                          )                                                                                      )                                                  +                                                      ∂                    2                                    ⁢                                                            (                                                                                                    X                            1                                                    ⁡                                                      (                            t                            )                                                                          +                                                                              X                            2                                                    ⁡                                                      (                            t                            )                                                                                              )                                        2                                                  +                                                      ∂                    3                                    ⁢                                                            (                                                                                                    X                            1                                                    ⁡                                                      (                            t                            )                                                                          +                                                                              X                            2                                                    ⁡                                                      (                            t                            )                                                                                              )                                        3                                                  +                …                                                                                        =                            ⁢                                                                    ∂                    1                                    ⁢                                      (                                                                                            A                          1                                                ⁢                        cos                        ⁢                                                                                                  ⁢                                                  ϖ                          1                                                ⁢                        t                                            +                                                                        A                          2                                                ⁢                        cos                        ⁢                                                                                                  ⁢                                                  ϖ                          2                                                ⁢                        t                                                              )                                                  +                                                      ∂                    2                                    ⁢                                                            (                                                                                                    A                            1                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ϖ                            1                                                    ⁢                          t                                                +                                                                              A                            2                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ϖ                            2                                                    ⁢                          t                                                                    )                                        2                                                  +                …                                                                                                      ⁢                                                                    ∂                    3                                    ⁢                                                            (                                                                                                    A                            1                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ϖ                            1                                                    ⁢                          t                                                +                                                                              A                            2                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ϖ                            2                                                    ⁢                          t                                                                    )                                        3                                                  +                …                                                                                        =                            ⁢                                                                    (                                                                                            ∂                          1                                                ⁢                                                  A                          1                                                                    -                                              0.75                        ⁢                                                                              ∂                            3                                                    ⁢                                                      A                            1                            3                                                                                              -                                              1.5                        ⁢                                                                              ∂                            3                                                    ⁢                                                      A                            1                                                                          ⁢                                                  A                          2                          2                                                                                      )                                    ⁢                  cos                  ⁢                                                                          ⁢                                      ϖ                    1                                    ⁢                  t                                +                …                                                                        (        b        )            where cos ωt represents the input signal x(t), αi is the gain at the i-th harmonic frequency of x(t), A1 and A2 are amplitudes of the neural signal and artifact, respectively. With a amplifier of a very low total harmonic distortion (THD) of 0.1% and a amplitude ratio of artifact-to-signal amplitude (A2/A1) of about 24.5, then y(t) is approximately 0, implying that the neural signal is lost as early as in the front-end amplifier stage. Moreover, in reality, the amplitude of the spikes are less than about 1 mV, while the artifact amplitude is typically ranging from tens (10s) to hundreds (100s) of mV.
Hardware approaches have also been reported to remove/reduce artifacts, such as blinking and electrode discharging. However, these approaches inevitably result in a signal loss since the amplifier cannot record a proper signal due to its saturation or input grounding during and shortly after stimulation. On the other hand, spectral cancellation of the artifact has also been implemented in the frequency domain, but this results in distortion as the spectrum of the signal of interest might overlap with that of the artifact.
Subtracting artifacts in the time domain, by creating an artifact template has also been adopted. This approach provides the potential to completely remove the artifact, yet this heavily depends on an effective means to acquire an accurate artifact template. This is further complicated by the intensity of the stimulation artifact, whose amplitude varies with distance between the recording electrode and the stimulation electrode. When attempting to use the same electrode for both neural recording and stimulation, a large artifact is inevitable, saturating the recording device. Equally important, in the process of acquiring stimulation artifact template, neural signals of interest must be exclusive.
In one attempted solution, a train of sub-threshold stimulus pulses are applied to acquire an artifact template and to avoid evoking actual neural responses. The template is then amplified based on the ratio of actual stimulus to sub-threshold stimulus for artifact cancellation. Nonetheless, a sub-threshold stimulus still effects the dynamic of neural tissue (for example, the threshold of excitability of the tissue is changed under sub-threshold stimulation). Electrode-tissue interface is also not a linear system, such that the artifact template acquired using sub-threshold stimulation cannot be generalized to present the practical artifact induced by larger stimulus intensity. In the above approach a certain period of time is further required in the training phase, preventing the scheme from adapting to different stimulus parameters swiftly.
A motion artifact is also an issue. Motion artifact is incurred due to the deformation of the soft biological tissue underneath the recording electrode or the change of contact area between the electrode and targeted tissue. This deformation or contact area change (e.g., skin stretch and relax, cortical tissue micro vibration, etc.) thus creates an undesired electrical potential change contaminating the signal of interest. For example, for a fixed amount of electrical charge accumulated on the human skin, the skin stretch would create a change of its equivalent capacitance, resulting in a voltage perturbation on the skin (i.e., motion artifact). Characterizing this impedance change in terms of its equivalent circuit model thus provides a motion artifact template that can be used to subtract the artifact from the empirical recording signals.
Accordingly, a need exists for a practical method and apparatus for real-time both stimulation and motion artifact suppression performed during simultaneous electrical stimulation and recording. The present disclosure overcomes the shortcomings of prior approaches while providing additional benefits.